“The Testament of Mary,” which opens tomorrow, presents a number of challenges for Shaw, including a plunge into cold water and shared stagetime with a vulture. Photo: Paul Kolnik

Even before the show begins, she’s onstage, sitting still as a statue in a see-through box. Invited to come up and see her, theatergoers gawk and sometimes kneel in front of her before they’re summoned to their seats.

By the time “The Testament of Mary” ends, some 90 minutes after that glass cage rises and disappears from the stage, the audience will have watched Fiona Shaw strip, bathe, dress, chop up a fish and exit with a vulture on her arm. And to think: Once the play opens tomorrow, she’ll be doing it six nights and two afternoons a week.

“Eight shows a week— who do they think I am?” Shaw says, laughing. “And what do they think I’m on?”

Backstage the other day in jeans and fraying sneakers, her hair still wet from the gym (“I thought it would dry if I walked on the sunny side of the street”), the 54-year-old was in high spirits. This despite a bad case of the sniffles she believes was caused by a plunge into the onstage pool of what, one night, turned out to be very cold water.

“I eat like a horse and drink like a fish,” she says. “I’d love to go till 4 in the morning having dinner and chatting, but this [show] takes a lot of preparation.”

That blend of the exalted and earthy — along with her high brow and narrow eyes — makes her fascinating to watch, either onstage and on screens large and small, in the “Harry Potter” flicks, in which she played Aunt Petunia, and cable TV’s “True Blood” (a woman who channels a witch).

Colm Tóibín says he turned his novella “The Testament of Mary” into the play with Shaw in mind, calling her “a mixture of the iconic and the ordinary.” Having seen her in Beckett’s “Happy Days” and “Medea,” the playwright says, “I knew what she could do.”

That “Medea” — directed, as “Testament” is, by the Irish actress’ longtime collaborator, Deborah Warner — played Broadway 11 years ago. So powerful was Shaw as a murdering mother that when she walked across the stage bearing the bloody bodies of her children, a man in the audience fainted.

Mary is another mother altogether. While the name Jesus is never uttered, we do see her suffer the loss of her only child, crucified and deified. The ferocious pain she feels when her son forcibly separates from her is something many women can relate to.

“I think Colm’s written the quintessential Irish mother in some ways, but I balk at that,” she says.

“Do you know that old joke — how many Irish women does it take to change a light bulb? None. ‘I’ll be grand here alone in the dark!’ ” and she bursts into a laugh. “I don’t think that woman is this woman! Mary isn’t a martyr. She has clarity, and she’s quite forthright.”

So is Shaw. This “doctor’s daughter from Cork” — her dad was an eye surgeon — grew up in a bookish family with three brothers. Fiona attended a school named after the Virgin Mary but “my mother kept me away from nuns.”

She left Ireland for England and the Royal Academy of Dramatic Arts, the springboard to what’s become a brilliant theatrical career punctuated by bread-and-butter projects like the “Harry Potter” series.

“It was just two weeks a year, not a huge lump of life,” she says of the “Potter” films. And while she doesn’t seem to miss the films, she regrets the passing recently of Richard Griffiths — “such a charming and talented man!” — who played the Mr. to her Mrs. Dursley.

And “True Blood,” the vampire hit in which Shaw’s character channeled a 17th-century witch?

“I loved doing it!” she cries. “HBO has become a sort of National Theatre of America — you’re working with brilliant people, everyone focused on the work.”

These days, she lives in London, not that she’s there much. She laughs when reminded that she’s described herself in the past as “happily single.”

“Who knows if it’s true?” she says. “Who knows when I’m happy? It doesn’t mean I’m not close to people.”

But onstage at the Walter Kerr Theatre, where “Testament” has its 12-week run. she’s all alone. Except, of course, for the vulture, whom she carries offstage early on.

The bird’s name is Pinhead, and no, she isn’t afraid of him. “The vulture won’t bite you unless you’re dead,” she says. “They only eat dead things.”

The play itself has been attacked for, among other things, questioning the divinity of Jesus, but she dismisses that as well.

“There was a little protest at the first preview, but they seemed to go away,” she says. “I wish the protesters would come see the show . . .

“It is not a play against religion. It’s a play about humans, and the ordinary tragedy of us all . . . Loss and grief but also optimism and hope and the beauty of memory. That’s what we’re all made of.”